I would recommend douching yourself out about 3-4 hours before you have start having sex. That way if there are any residual water left, it will be absorbed. Trust me, I learned from that messy experience once. Douching right before anal sex = not good idea.
Also since this will be your first time, make sure you use a dildo (small at first) to help you get used to the sensation of having something in your rectum. The more prep you do, the more enjoyable it will be. Yes, lots of lube is key. Try riding him first because you have control of the rate and rhythm of the penetration.
As for diet, try not to eat much diary products that day.
Anal sex should not be painful with adequate preparation and stimulation (perhaps have him rim you first) Have fun!
